Spatial light modulators are in wide use in displays systems and are increasingly being used due to having the benefit of high resolution while consuming lower power and less bulk than conventional Cathode Ray Tube (CRT) technology. One type of SLM display is the Digital Micro-Mirror Device (DMD). The DMD device typically consists of an array of small reflective surfaces, mirrors, on a semiconductor wafer to which electrical signals are applied to deflect the mirrors and change direction of the reflected light applied to the device. A DMD-based display system is created by projecting a beam of light to the device, selectively altering the individual micro-mirrors with image data, and directly viewing or projecting the selected reflected portions to an image plane, such as a display screen. Each individual micro-mirror is individually addressable by an electronic signal and makes up one “display element” of the image. These micro-mirrors are often referred to as picture elements or “pixels”, which may or may not correlate directly to the pixels of an image. This use of terminology is typically clear from context, so long as it is understood that more than one pixel of the SLM array may be used to generate a pixel of the displayed image.
Generally, projecting an image from an array of DMD pixels is accomplished by loading memory cells connected to the pixels. Once each memory cell is loaded, the corresponding pixels are reset so that each one tilts in accordance with the ON or OFF state of the data in the memory cell. For example, to produce a bright spot in the projected image, the state of the pixel may be ON, such that the light from that pixel is directed out of the SLM and into a projection lens. Conversely, to produce a dark spot in the projected image, the state of the pixel may be OFF, such that the light is directed away from the projection lens.
Modulating the beam of light with a micro-mirror is used to vary the intensity of the reflected light, such as through Pulse Width Modulation (PWM). Although the micro-mirrors can be moved relative to the bias voltage applied, the typical operation is a digital bi-stable mode in which the mirrors are fully deflected at any one time. Generating short pulses and varying the duration of the pulse to an image bit changes the time in which the portion of the image bit is reflected to the image plane versus the time the image bit is reflected away, therefore distributing the correct amount of light to the image plane.
The above-described pulse-width modulation techniques may be used to achieve varying levels of illumination in both black/white and color systems.
For generating color images with SLMs, one approach is to use three DMDs: one for each primary color of red, green, and blue (RGB). The light from corresponding pixels of each DMD is converged so that the viewer perceives the desired color. Another approach is to use a single DMD and a color wheel having sections of primary colors. Data for different colors is sequenced and synchronized to the color wheel so that the eye integrates sequential images into a continuous color image. Another approach uses two DMDs, with one switching between two colors and the other displaying a third color.
A PWM scheme is determined by using the display rate at which images are presented to the viewer and the number of intensity levels available by the display system. The display system rate gives the time that the image frame is available for viewing. For example, a standard television signal is transmitted at 30 frames per second (fps), which is a frame time of 33.3 milliseconds. For a system having n bits of resolution, the image has 2n levels of intensity. Thus, if the system has 4 bits of intensity resolution, there could be 16 levels of intensity. To create the perception of an intensity level, in PWM systems, the frame is divided into equal time slices, which will display a quantized intensity. For a system having n bits of intensity resolution, the frame is divided into 2n−1 equal time slices. After the image element intensity is quantized, a black value, 0, would contain no intensity and be equivalent to 0 time slices while the maximum brightness level would have the display element on for all of the time slices, or 2n−1 time slices.
An established method to get the time slices into a display frame is to format the data into “bit planes” where each bit-plane corresponds to a bit weight of the intensity value. A system with the 4 bits of intensity resolution would have 4 bit-planes and each bit-plane would be weighted with appropriate time slices. An example would be that the 21 bit or least significant bit (LSB) would have one time slice, the 22 bit would have two time slices, the 23 bit would have 4 time slices, and the 24 bit or MSB would have 2n/2 or 8 time slices. By displaying all of the bit-planes within a frame, any of the capable intensity levels can be created in this weighted method. Bit-planes may be displayed in various orders. The bit-plane that only represents one time has the shortest “on” time for the display elements, and the time to load this LSB bit-plane is the “peak data rate.” Since SLM display systems typically have many display elements and since the desired intensity levels are higher than the example above, the data rates to get the intensity information to each display element can become very high.
U.S. Pat. No. 5,278,652, entitled “DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System,” assigned to Texas Instruments Inc., describes various methods of addressing a DMD in a DMD-based display system. These methods are directed to reducing the peak data rate while maintaining optical efficiency. Some of the methods discussed therein include clearing blocks of pixel elements and using extra “off” times to load data. In one method the time in which the most significant bit is displayed is broken into smaller segments so as to permit loading for less significant bits to occur during these segments.
Another method of reducing the peak data rate is referred to as “memory multiplexing” or “split reset.” This method uses a specially configured SLM, whose pixels are grouped into reset groups that are separately loaded and addressed. Although this increases the complexity of the device, the method reduces the amount of data to be loaded during any one time and permits the LSB data for each reset group to be loaded at a different time during the frame period. This configuration is described in U.S. patent application Ser. No. 08/002,627, which is commonly assigned to Texas Instruments Inc. with the present disclosure.
PWM methods can result in the display of visual artifacts that the viewer can perceive. Regardless of whether or not the pixels of the SLM are addressed all at once or are multiplexed, visual artifacts should be minimized. “Temporal contouring” is a type of artifact possible with bit-plane data when a number of “ON” states occur closely with a number of “OFF” states. As an example, for an 8-bit system, if in one frame a pixel has an intensity level of 128 and the most significant bit (MSB) display time occurs during the first half of the frame time, the pixel is ON for this length of time and OFF for the rest of the frame time. If, in the next frame, the pixel's intensity is 127, the pixel is OFF for the MSB time and ON during the display time for all other bits during that frame. The point in time when all bits change state can cause a visual artifact, which is more perceptible as brightness increases. Thus, artifacts such as these temporal artifacts are undesirable and should be removed from the final displayed image.